This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This clinical trial will evaluate the safety and tolerability of an investigational drug, GT4P, compared to Buphenyl in patients with urea cycle disorders (UCDs).